Solid oxide fuel cells (SOFC) and other similar energy conversion devices are often utilized at extreme operating conditions and will frequently experience integrity problems, primarily due to the different materials used within the energy conversion device.
For instance, FIG. 1A sets forth a typical energy conversion device 10 in which the electrolyte 12 and the cathode 14 are formed of different materials. For example, the electrolyte 12 can be cerium(IV)oxide (CeO2) as is known in oxygen ion systems. The electrolyte ideally serves to transportionic species without significant levels of electronic conductivity so as to avoid deterioration of the device performance. The material of the cathode 14 is a porous, mixed ionic and electronic conducting material such as (La,Sr)MnO3 that allows for both oxygen ion and electron transfer. Solid oxide fuel devices as illustrated in FIG. 1A can also include an anode (not shown in FIG. 1A), e.g., a nickel/ceramic composite, that is porous to allow gas transfer and is stable at low oxygen partial pressure conditions.
Solid fuel devices typically operate at elevated temperatures and are subjected to extreme temperature fluctuations during the heating and cooling steps necessary to achieve a steady state operation. These operation conditions lead to difficulties during operation. One difficulty is inter-diffusion and secondary phase formation at the interfaces between the electrolyte and the cathode. This secondary phase formation degrades the cell performance. A second difficulty concerns cracking and mechanical failure that is contributable to differences in thermal expansion of the materials used to form the cathode, electrolyte, and anode of solid oxide fuel cells.
Other solid fuel cells such as polymer electrolyte membrane (PEM) fuel cells also require electronically conducting layers that resist degradation over long term use. For instance, a significant problem hindering large-scale implementation of PEM fuel cell technology is the loss of performance during extended operation and cycling. Investigations of the deterioration of cell performance have revealed that a considerable part of the performance loss is due to the degradation of the electrocatalyst. High surface area carbon materials such as carbon blacks have found widespread use as a catalyst supports for fuel cells owing to their low cost, good electron conductivity, high surface area, and chemical stability.
While carbon has been proven to be a stable support in the anode environment, it has been shown to be thermodynamically unstable in the cathode environment. When exposed to water and oxygen the carbon supports react to form carbon dioxide (CO2). Sintering and dissolution of the catalyst, generally platinum and platinum alloy, on the carbon surface increases with carbon corrosion. Oxidation of the carbon surface increases hydrophilicity and affects water removal leading to increased mass transport losses. Also, carbon corrosion decreases the thickness of the catalyst layer leading to decrease in the electrical contact between the current collector and subsequent increase in the cell resistance.
The high surface area and surface chemistry of carbon play an important role in the deposition and stability of catalystic metals such as platinum (Pt). Unfortunately, the very properties that make carbon materials good catalyst supports (i.e., high surface area and surface functionality) serve to enhance corrosion at high oxidative potentials. The instability of carbon is accelerated at the current operating conditions of the fuel cell such as high metal loadings (e.g., greater than 10 wt %), high water content, low pH (generally less than 1.0), high temperature, high oxygen concentration, and high operating potentials, particularly those that occur during startup and shutdown cycles.
Accordingly, there remains room for improvement and variation within the art of energy conversion devices. For instance, what are needed in the art are energy conversion devices that provide long term use without degradation due to, e.g., secondary phase formation, disparate thermal expansion characteristics, corrosion, and the like. For instance, the ability to produce highly dispersed and high surface area Pt crystallites is essential to increase the reaction rate of kinetically slow reactions such as the oxygen reduction reaction in a PEM fuel cell. Thus, there exists the need to develop catalytic support materials that afford improved corrosion resistance. Increased corrosion resistance of the catalyst support is required in order for fuel cells to meet the design lifetime requirements of 40,000 hours for stationary applications and 5,000 hours for transportation applications.